Process for manufacturing metal articles



atented Sept. e, ihdfi PROCESS FOR MANUFAC'EING T ARTICLES William David Jones, London, England No Drawing Application May 3, 1938, Serial No. 2053M. in Great Britain May 3, 1937 12 Claims.

This invention relates to metal articles and to a process for the manufacture thereof.

It is known that porous base metal articles can be manufactured by pressing together metal a powders and subsequently heating them. Such articles have been porous-throughout, i. e. they have had pores on the surface as well as in the interior and have been used for various purposes,

for example, as oil-less bearings. lamp wicks, ink in blotters, and the like.

Itis an object of this invention to, provide precious metal articles of low weight per unit volume, but of the same appearance to the 'naked eye as ordinary precious metal articles.

. It is a further object of this invention to provide a process for the manufacture of such articles. Y

According to this invention precious metal articles are provided which have pores in the interior but nopcres which are visible to the naked eye on the surface thereof.

According to the process of this invention for the manufacture of precious metal articles having pores in the interior but no pores which are 5 visible to the naked eye on the surface thereof a precious metal powder is subjected .to a heat treatment to cause the particles of powder to sintertogether whereatter the surface of the article is, if necessary, subjected to a mechanical or thermal treatment to remove pores therefrom.

\ The term precious meta as used herein means one of the group of metals, gold, silver,

platinum, palladium, rhodium, iridium, ruthenium, and osmium, or mixtures thereof, or alloys 3 in which one or more of these metals is, or are the predominating constituent, or constituents. I

It will be understood that when alloys are used they may be made from .powders by com-' mencing either with the powdered alloy or with a 40 the alloy is made from a mixture, as for example, the manufacture of "standard silver from a mixture ofcopper and silver powders, then the heating or .-sintering treatment must be sumciently extended to allow diffusion and alloying to take place. This may take several hours if the .temperature is below the melting point of any of the alloys formed, but may be shortened to less than an hour if a fusible constituent is K generated. For example standard silver may be made by taking the correct mixture of 92.5% by weight of silver powder and 7".5% by weight of copper powder, pressing them together in a suitable mould and-heating the compact to a i temperature of 800 C. in an atmosphere of mixture of the alloying elements or both. If

'si'ons during its heat treatment.

hydrogen. At this temperature a small propor= tlon of fusible eutectic is formed and alloying quickly takes place so thatlf the original powders were finer than 300 I. M. M. mesh in the first instance it is only necessary to extend the heat ing time to 30 minutes. At a temperature oi 750 C. more than 3 hours would be necessary. In the manufacture of a pure metal compact there is no necessity to allow time for diffusion and the heating time may be as short as practicable.

The manufacture of articles of precious metal alloys generally requires the heating to be at iected in a reducing or protective atmosphere since standard silver and copper-gold alloys and the like oxidise to an appreciable extent if heated to a high temperature in air.

As stated hereinbefore the articles of the present invention have. pores in the interior thereof and this results in a reduction of density below that of the normal cast or Worked metal or alloy and in addition the following require-v ments should preferably be complied with.

(I) That the pores shall be of such a size that they are not visible to the naked eye in any part of-the article so that it is possible to saw the article in half and the pores cannot even then be seen. It is further desirable that the pores shall not even be seen under the microscope after normal operations of cutting and polishing or buffing. (II) That the article can be pressed to the final shape in one operation so that no further finishing or shaping operations have to be effected, (III) That blank articles can be made which if desired can be shaped as required, and it is therefore desirable that the material shall be susceptible of being shaped by the normal operation of cutting, spinning, rolling, and of soldering treatment or the like without any remarkable subsequent change in density, it being always understood however that the application of high pressures or high temperatures at any time subsequent to manufacture will naturally increase the density. (IV) That articles can be manufactured to size with .precision. This requirement necessitates either controlled shrinkage or expansion during heat treatment, or alternatively, which is better, that the pressing shall-not change in external dimen- (V) That on the grounds of cost of manufacture the pressures used in pressing the powders in the first instance shallbe sufllciently low to permit of the use of cheap die materials for example of fusible metals or Bakelite or wood. When any very iii large quantity of articles of a given shape are to be manufactured this requirement is not so important and the cost of an engraved steel die becomes relatively low in comparison to the cost of the metalalthough of course the use of low pressures leads to a longer life for the die set. (VI) Further'to decrease the cost of manufacture the metal powders employed as initial materials should not cost appreciably more than the metal of corresponding purity would cost in the normal cast condition. It is for example. useless to make articles of half the normal density in order to reduce theselling priceif the cost of the starting powder is more than double the cost of the solid metal. The successful working of this invention therefore 11.608551 tates cheap methods of manufacturing precious metal powders.

All these requirements can be satisfied by the methods disclosed herein although in some cases where thse requirements are opposed to themselves it is necessary to make some compromises.

According to an embodiment of the process of this invention precious metal powder is mixed with a volatile substance such as stearic -acid, or salicylic acid and subjected to a heat treatment to cause the particles of powder to sinter together and cause the volatile substance to volatilise and form pores, whereafter the surface of the article is subjected to a" treatment, for example, burnishing or a heat. treatment which causes momentary fusion, or semi-fusion -of the surface without substantially affecting the interior, to remove the pores from the surface.

According to a further embodiment of the process of this invention precious metal powder is introduced into a heat-resistant box, or like containerin which it is heated at moderate temperature under low pressing pressure, a fine powder being employed for the surface layers. If there is any porosity at the surface thisis re-- moved, for example, by one of the methods hereinbefore described.

The porosity can be of any desired degree from almost complete solidity to or by volume, but it is preferably between 15% and 60% by volume.

The size of the pores is generally maintained sufliciently small that a cross section of the material shows pores not visible to the naked eye although the pores can be coarser inside if desirable.

Control of porosity can be effected by careful selection of the particle size or range of particle'sizes of the powder, pressing pressure, and temperature of heat treatment. low pressing. pressures and moderate temperatures for theheat treatment it is possible to obtain considerable porosity.

surface of the articles unless they are subjected to an after-treatment to remove the pores from the surface although coarser powders may be used if desired for the interior. Very fine silver and gold powders may be made in many ways such as for example bychemical precipitation and by reduction of asalt in a reducing atmosmal powders of commerce.

By the .use of gamma phere at a high temperature, or by electrolytic precipitation using high current densities. All these powders are-suitable for use and are nor- With the exception of the electrolytically precipitated powders-they are however rather expensive to manufacture and sell usually at considerably higher prices than the cast metal. These fine powders also generally have the disadvantage that they shrink to a very large extent when being sintered. Shrinkages of 10 to 25% in linear dimensions are not uncommon for example with precipitated silver. This is not of great importance when blank articles are to be produced but this shrinkage is too large to manufacture to size with precision. Generally shrinkage can be reduced by increasing the initial pressing pressure, and with precipitatedsilver it is for example possible to sinter the compact without change in size at 800 C. by, pressing the powder at pressures of the order of 50 tons/sq. in. Such pressu'res whilst by no means impossible are, however, inconvenient. The use of high pressures of this type generally leads in addition to finished articles of too high a density. Shrinkage can also be reduced by the addition to the powder before pressing of a material which will volatilise and evolve gases during sintering. For example the addition of to 10% by weight of paraflln, shellac, stearin, fuel oil, ammonium carbonate, silver carbonate, silver citrate, silver succinate, tartrate, or oxalate, all dissolved in a suitable solvent to facilitate mixing if necessary, will bring about modification of shrinkage during sintering. When sintering takes place in a. reducing atmosphere then it is frequently possible to add oxides such as silver or copper oxides which; during reduction, will by the production of steam alter the dimensions of the hot com-- pact. Generally however it is not possible to do very much by such additions since any large amount of gas forming substances will often lead to the formation of blisters or bubbles on the finished article. Modifications in the shrinkage characteristics'of a powder can often be obtained by heat treating it before sintering. For example heating precipitated silver powder to 400 C. considerably changes. its characteristics. Again it is not possible to go very far in this direction since heating of the powder fora long time or at a high temperature will obviously tend more convenient to make use of a powder of.

gold or silver which when pressed in a die to form a compact and heated to the sintering temperature will expand to some extent. Such expansions can then be neutralised by promoting the formation of a small proportion of a liquid phase during sintering as for example, a small amount of silver-copper eutectic.- It was found that some types of electrolytically precipitated powders satisfy this requirement. For the preparation of a standard silver article to specific dimensions I prefer to use a silver powder which has beendeposited cathodically from a silver nitrate electrolyte using moderate amounts of nitric acid, low current densities, and normal temperatures. For example, using a bath containlng 3% of silver nitrate, 1% of nitric acid, operated at a cathode current density of 30 amperes per square foot, at room temperatures,

I can obtain, by scraping the cathode every ten minutes, a fine silver powder, which will expand considerably on being pressed and sintered. This process of producing the silver is advantageous inasmuch as by increasing the current density or making adjustments to the composition of the bath I can effect a considerable control of the characteristics of the powder, and I can if necessary produce an extremely fine powder suitable for the facing of my pressed articles. This process also has the necessary requirements of cheapness since the manufacture of the powder can be combined with the normal processes of gold or silver refining and it is therefore possible to commence powder manufacture with impure gold or silver bullion. Powder manufacture by such methods is very simple in the case of gold or silver as distinct from copper for example where it is found necessary to control thebath by the addition of various colloids such as gelatine.

The following examples illustrate how the process of this invention may be carried into effect:

1. A 22 carat gold medal was. made from powders by pressing in a mould obtained by casting a lead-tin-antimony alloy around a plaster pattern. For the major part of the medal approximately powers of to mesh 22 carat gold were employed, whilst for the surface 600 mesh 22 carat gold was employed.

The coarse powder was a hard electro-deposited powder whilst the fine powder was of a soft spongy texture obtained by chemical precipitation.

After pressing at ton/sq. in. the article was heated in a reducing atmosphere in an electric furnace for 15 minutes at 785 C. and after cooling the surface was lightly bufied. The finished medal had a density of approximately 10.4 gms. per c.c., whereas the density of cast 22 carat gold is about 17.35 gms. per c.c. To the naked eye the medal presented the normal appearance of massive metal.

2. A mixture of 92.5 parts-by weight of 500 mesh silver powder made by the electrolytic method described above and 7.5 parts by weight of a 500 mesh copper powder, was'prepared by prolonged mixing, and the mixture was pressed in a die which has been made bythe casting of a fusible metal around a pattern of the article to be manufactured. Pressing was effected under a pressure of the order of 2 tons/sq. in. Sintering was effected for 30 minutes at 800 C. in a high frequency induction furnace provided with an atmosphere of hydrogen.

The density of the finished articles when made from standard silver are preferably between 5.0 and 8.0 which is the most suitable range for most purposes. If it is desired to increase the density of the article, I use either a slightly high temperature for sintering, a longer time of sintering, or a slightly different powder made by adjusting the conditions of electrolytic deposition as for example, by increasing the current density. Any of these alterations willcause a slight shrinkage to take place during sintering (assuming that the conditions had been adjusted that no change in lmear, dimensions occurred before the alterations were made). Such a shrinkage can then in this case be neutralised by slightly increasing the pressing pressure, by heat treating the powder before pressing, or by suitable volatile additions as hereinbefore described. Similarly-reductions in density can also be effected, and similarly slight expansions can also be controlled by the manipulation of the various factors mentioned. I may of con 5e use a facing of a slightly finer powder on the article in order to produce a better surface appearance and I may if desirable improve the appearance by subsequent bumng, burnishing, colouring, polishing, or incipient surface fusion. An article made in this manner will have a porosity which will be too fine to be seen with the naked eye on a cut or sawn surface,

'and even if a cut or filed surface is polished andexamined under the microscope no porosity will be seen since the operation-of polishing causes a slight fiow of the metal which will obscure the very fine pores. The surface of the article will however be free from porosity made under these conditions since the heat treatment causes a filling up of the surface pores, presumably under the operation of surface tension forces, or in any case can be made non-porous by the methods described.

As indicated hereinbefore the process of the invention is not limited to the use of hydrogen as an atmosphere for sintering. Any atmosphere may be used which is industrially used for the heat treatment of such alloys such as for example, partially burnt and purified coal gas, cracked, or partially burnt ammonia gas, propane, butane, or the heat treatment may even be effected in vacuo, or in such gases under very low pressures, or even in air but protected from oxidation by being surrounded with reducing substances such as charcoal.

The density of the articles obtained can be modified in subsequent manufacturing treatments by pressing, or otherwise deforming the article either at room temperatures or at elevated temperatures, and it will be understood that such subsequent manufacturing treatments may employ pressure at elevated temperatures.

The articles may according to the invention consist of various precious metal alloys in different parts of the object. Thus for example astandard silver article may be made with a porous or non-porous gold alloy insertion or inlay, or vice versa, orwith a base metal insertion not necessarily made from powders or if made from powders, not necessarily made at the same pressing operation. Thus for example a porous standard silver knife handle can be pressed around the shank of a steel knife.

It is obvious that by suitable manipulation of the conditions of manufacture it is possible to make articles of varying density in different parts of the same article.

Substances may be incorporated in the powders which evolve reducing gases for the purpose of producing the necessary reducing atmosphere internally and thus permit sintering to be efiected.

can be made.

As indicated hereinbefore, the articles of this invention are lighter than ordinary precious metal articles and consequently the selling Price can be reduced.

An advantage of the process of this invention is that owing to the possibility of using low press lng pressures, relatively weak mould materials may be employed. Thus, for example, in the stamping of gold medals it is necessary tame a hard tool steel for the die or mould, and the engraving of this material may be an expensive matter. of this invention, however, the moulds may be made of much weaker materials and there is no need for them to be metallic. Casting of the mould to shape can therefore replace the more expensive engraving operation. Such moulds may be of wax, cement, plaster, low melting point alloys, or even of wood, or rubber compositions.

The mould may also be made by electrodeposition from a wax or plaster pattern in a similar manner to that employed in the manufacture of gramophone records.

What I claim is:

1. A process for the production of porous prec lous metal articles 'of a density lower than that of the metal in cast condition, which comprises introducing a precious metal po der into a container in such a way that at 1 st the parts in the immediate vicinity of the w 11s of the container consist of particles finer than 400 mesh, subjecting the powder to a low pressing pressure to produce a compact, and heating the compact to cause the particles of powder to sinter together, the characteristics of the powder and the nature of the heating being such that no substan tial change in the dimensions of the compact takes place during slntering and the article formed has pores in the interior thereof but no pores which are visible to the naked eye on the surface thereof.

2. A process for the manufacture of porous precious metal articles of a density lower than that of the metal in cast condition, which comprises subjecting a mass of a mixture of precious and non-precious metal powders in which the precious metal powder predominates, at least the Juter layer of the mass consisting of particles finer than 400 mesh, to a treatment involving heat and pressure to cause the particles of the powders to sinter together, the characteristics of the powder and the nature of the heating being such that alloy formation'takes place and so that the article formed has pores in the. interior thereof but no pores which are visible to the naked eye on the surface thereof.

3. A method of manufacturing porous precious metal articles of a lower density but of the same appearance as ordinary precious metal articles, comprising subjecting a mass of precious metal powder, at least the outer layer of which consists of particles finer than 400 mesh, to a'pressure of V to 2 tons per square inch, and heating the pressed article to a temperature high enough to sinter the powder particles together,.but below the melting'point of the metal, to produce a porsus article, the surface of which appears impervious. r

4. A-method of manufacturing porous precious metal articles of a lowerdensity but of the same appearance as ordinary precious metal articles, :omprising subjecting a mass of precious metal powden capable of expanding when sintered and at least the outer layer of which consists of par- ;icles finer than 400 mesh, to a pressure of to 2 tons per square inch, and heating the pressed When working according to the process comprising subjecting a mass of precious metal powder, at least the outer layer of which consists of particles finer than 400 mesh, to a prelim- I inary heating below the sintering temperature to reduce the tendency of the mass to shrink upon sintering, pressing the mass at a pressure of /2 to 2 tons per square inch, and heating the pressed article to a temperature high enough to sinter the powder particles together, but below the melting point of the metal, to produce a porous article, the surface of which appears impervious.

6. A method of manufacturing porousprecious.

metal articles of a lower density but of the same appearance as ordinary precious metal articles, comprising subjecting a mass of precious-metal powder, at least the outer layer of which consists of particles finer than 400 mesh, admixed with a substance capable of producing a gas when heated, to a pressure of to 2 tons per square inch, and heating the pressed article to a temperature high enough to sinter the powder particles together, but below the melting point of the metal to produce a porous article, the surface of which appears impervious.

7. A method of manufacturing porous precious metal articles of a lower density but of the same appearance as ordinary precious metal articles, comprising subjecting a mass of precious metal powder, at least the outer layer of which consists of particles finer than 400 mesh and which is produced electrolytically under such conditions of temperature, current density and electrolyte compositions as to have the property of expanding when sintered, to a pressure of V2 to 2 tons per square inch, and heating the pressed article to a ing of gold powder of about 600 mesh to a pressure of about /2 ton per square inch, and heating the pressed article at 785 C. for about 15 minutes.

9. A method of manufacturing porous precious metal articles of a lower density but of the same appearance as ordinary precious metal articles, comprising subjecting a mass of approximately powders of -to mesh 22 carat gold and a surfacing layer of 600 mesh 22 carat gold powder to a pressure of /2 ton per square inch, and heating the pressed article at 785 C. for about 15 minutes. 1

10. A method of manufacturing porous precious metal articles of a lower density but of the same appearance as ordinary precious -metal-articles, comprising subjecting a mass of 92.5' parts by weight of 500 mesh silver powder admixed with 7.5 parts by weight of 500 meshcopper powder to a pressure of about 2 tons per square inch, and slntering the pressed mass for. 30 minutes at 800 C. in a, reducing atmosphere.

11. A method of manufacturing porous precious metal articles of a lower density but of the same appearance as ordinary precious metal articles,

that of the metal in cast condition, which comprises subjecting a mass of precious metal powder at least the outer layer of which consists of particles finer than 400 mesh to a treatment involving heat and pressure to cause the particles of powder to sinter together so that the article formed has about 15 to 60% pores in the'interior thereof but no pores which are visible to the naked eye on the surface thereof.

'DAVID JONES 1o 

